This invention relates to a method for detecting the speed of a rotating member such as the wheel of a vehicle. Typically in order to sense the rotational speed of a rotating member, such as the wheel of a vehicle, a tachometer is provided that generates a signal having a frequency directly proportional to the rotational speed. The tachometer may take the form of a toothed ring rotated by the rotating member, the rotation of the ring being sensed by an adjacently located electromagnetic sensor. The electromagnetic sensor provides a pulse each time the toothed ring, and hence the vehicle wheel, rotates 1/N of one revolution where N is the number of teeth on the toothed ring. Each pulse may directly comprise the speed signal or alternatively may be shaped into a square wave speed signal. Given the relationship between the rotation of the toothed ring and the rotation of the vehicle wheel, the number of speed signals generated during a fixed time interval is directly proportional to the rotational speed of the rotating wheel. Similarly, the elapsed time between each of successive speed signals is inversely proportional to the rotational speed of the wheel.
There are many known methods for processing the speed signal. In one known method, described in U S Patent number 4,799,178 assigned to the assignee of this invention, the occurrence time of each speed signal and the number of speed signals generated during a fixed period of time are recorded. Utilizing the aforementioned relationships between a) the elapsed time between successive speed signals, b) the number of speed signals generated during a fixed sampling interval and c) the rotational speed of the rotating member, the rotational speed of the wheel is calculated at the end of each fixed sampling interval using the number of speed signals counted during the current sampling interval, the occurrence time of the last counted speed signal of the current sampling interval and the occurrence time of the next-to-last counted speed signal of the previous sampling interval. In this method the accuracy of the calculation is maximized by extending the calculation interval over a time greater than the sampling interval.
Typically, once the digital computer calculates rotational speed, that calculated speed is utilized as a variable in a vehicle control algorithm. For example, an antilock brake control system can use wheel speed to determine wheel slip and wheel acceleration. These parameters are then monitored, and wheel brake pressure is cyclically decreased and increased to avoid incipient wheel lock and provide higher braking efficiency and improved vehicle stability. The primary task for an antilock brake computer, therefore, is the modulation of wheel brake pressure, while the computation of wheel speed is an ancillary, albeit necessary, task.
Speed signal processing efficiency is a measure of the amount of time expended by the calculation device, such as a digital computer, in the calculation of rotational speeds versus the amount of time expended by the device in the performance of its other tasks. Since counting speed signals and recording the occurrence times for these signals involves a fixed amount of processing time per speed signal, processing efficiency using solely the method disclosed in US Patent 4,799,178 decreases as the number of speed signals being utilized in the speed calculation increases. Conversely, however, calculation accuracy increases as the amount of data (i.e. the number of speed signals) utilized in the speed calculation increases. Simply stated, the recording of edge occurrence times for every speed signal received while counting the number of speed signals received requires a great deal of processing time. This processing time in turn diverts the attention of the digital computer from its other functions and thereby sets an effective limit of processing capability. This limit is directly proportional to the rotational speed of the rotating member, and hence is directly proportional to the frequency of the occurrence of speed signals. Beyond this limit, the digital computer devotes too much processing time to the calculation of rotational speed to adequately service its other critical tasks. Speed processing efficiency could be increased by reducing the number of speed signals generated by the tachometer (i.e. fewer teeth of the toothed ring). However, that would sacrifice calculation accuracy. Similarly, the fixed sampling period could be decreased. But that would result in lower calculation resolution. Ideally, to maximize accuracy as well as processing efficiency, a speed calculation method should count and record the occurrence time of only those speed signals necessary to the calculation without compromising the accuracy or resolution of that calculation.